By opening a six-tonne freight delivery intended for the British mission in Harare, the Zimbabwean government stands accused of flouting diplomatic law.

According to international convention, packages carrying official documents and other material deemed necessary for use by a diplomatic mission are "inviolable".

The so-called diplomatic bag, provided that it is clearly marked as such, cannot be opened or detained.

Passports are just some of the diplomatic baggage

Just as diplomats and their embassies are exempt from the rules and regulations which govern others in a host nation, their correspondence is also above national law.

Any items entering a host country in this way are not within the jurisdiction of customs officials or subject to import taxes.

Enshrined in the 1961 Vienna Convention on Diplomatic Relations, the status of the diplomatic bag allows the UK to send "anything we want" to our embassies, according to Foreign Office spokesman Richard Wood.

However, certain items, such as the arms the Zimbabweans claim they were searching for, are not permitted, says Mr Woods.

The Vienna convention also gives special status to the couriers who accompany the diplomatic mail.

Churchill: "Cigars? Check my bag."

In the case of the UK, these Queen's Messengers cannot be hindered in their duties. They are liable to neither arrest nor detention.

For larger consignments, such as the one sent to Harare, packages are loaded into an aeroplane cargo hold in the UK and met by diplomatic officials at their destination.

While much information is now sent from the Foreign Office to its overseas missions via satellite, the diplomatic bag remains a lifeline for embassy staff.

Personal possessions sent in the bag are only subject to customs inspections once back in the UK.

Stocks of documents such as passports, vital to the everyday running of a mission, also come via this route.

In the bag: The Wi-le Saxon

Each year the Queen's pre-recorded Christmas Day message is sent in the diplomatic bag to every corner of the world.

During World War II, Prime Minister Winston Churchill was kept in Cuban cigars thanks to the bag and a wealthy well-wisher in New York.

Following China's cultural revolution, the Foreign Office received an interesting memento of the turmoil from the embattled staff of the Beijing embassy.

Cocking a snoot at the besieging Red Guard, British charge d'affaires Donald Hopson cut down one of the straw effigies of Prime Minister Harold Wilson hung on the mission building and put it in the diplomatic bag.

A placard around the neck of the likeness read: "Down with Wi-le Saxon!"